The present invention relates to a packing container of laminated material comprising a seal wherein a first edge area of the laminate is sealed to a second edge area, the side of the seal facing towards the inside of the packing container being covered by means of a liquid-tight material strip. The invention also relates to a packing laminate web for production of the container.
Packing containers for liquid foodstuffs are generally manufactured from the laminated material which comprises a central carrier or base layer of paper or cardboard which is covered on both sides with a thermoplastic material, e.g. polythene. The paper layer in this case has a carrying function, whilst the thermoplastics on the one hand makes the material impermeable to liquid and on the other hand makes possible a heat-sealing of the material without a supplementary adhesive.
A known packing container for milk is manufactured in that a laminated material of the aforementioned type is fed to a packing machine in the form of a web. In the machine the material web is converted to tubular shape, in that the longitudinal edge portions of the material web are made to overlap one another, whereupon they are heated so that the thermoplastic layers reach their softening temperature and are compressed to a longitudinal liquid-tight overlap joint. The filled tube is then converted to a line of connected, cushion-like containers in that the tube at uniform intervals is compressed and sealed in transverse zones. The cushions formed, wholly filled with contents, are subjected moreover to a forming process in the course of which forming jaws give the desired shape, e.g. parallelepipedic shape, to the packages.
The longitudinal overlap joint mentioned above is formed in a known manner in that the two edge areas of the material are placed on top of one another and are heated and compressed to a liquid-tight seal. One edge area will then be located inside the finished container, which means that the cut edge remains unprotected so that the central paper layer of the laminate will come into contact with and absorb some of the contents. To prevent this, the seal is generally provided with a so-called longitudinal joint strip, that is to say, a liquid-tight material strip of the same material as the plastic layer of the laminate, e.g. polythene, which strip, after the sealing is applied over the edge area situated inside the packing container in such a manner that the cut edge itself is sealed off from the interior of the package.
When the packing container described is to be used for the packing of sterile contents, the packing laminate is sterilized with the help of a sterilizing agent, e.g. hydrogen peroxide, which is applied to the packing laminate. After a certain time in contact with the packing laminate the sterilizing agent is removed again, which is done among other things by heating the inside of the packing material tube so such a temperature that the sterilizing agent is evaporated and can be drawn off by ventilation. Since the outside of the material tube during the heating is exposed to a certain amount of cooling by the surrounding air, the packing laminate will not be heated to temperatures which are harmful for the laminate. In the area of the longitudinal joint of the tube, however, where the edge zones overlap one another, the material is so thick that the cooling will be appreciably impaired. This means that the sealing zone is subjected to such a high temperature that the sealing strip commences to melt at the same time as the natural moisture of about 6% enclosed in the paper layer of the laminate is made to evaporate, which is true in particular for the edge area located close to the centre of the material tube. Since the cut edge of the edge area is enclosed underneath the sealing strip, the space underneath the strip will be filled with expanding vapour until the sealing strip, softened up by the heat, is deformed or breaks, so that the vapour can escape from the space between the sealing material strip and the laminate.
Up to now it has been tried to eliminate the difficulties described by designing the sealing material strip in such a manner that the vapour cannot penetrate the same. In a known solution a laminated strip is proposed which had an enclosed core of heat-resistant material, e.g. HD-polythene. This strip has certainly proved to function relatively well, but the manufacture of the strip is complicated and the strip becomes relatively expensive.
It is an object of the present invention to overcome the abovementioned problem and to provide a solution which allows the utilization of an uncomplicated and inexpensive longitudinal joint strip.
This object has been achieved in principle in accordance with the invention in that the longitudinal seal is designed in such a manner that the vapour can be drawn off by ventilation without affecting the sealing strip, and more particularly, the abovementioned object has been achieved in that a packing container of the type described in the introduction has been given the characteristic that ducts are arranged to connect a space present between the material strip and the laminate with the outside air. The ducts make possible a ventilation of the previously closed space underneath the sealing longitudinal strip, as a result of which the vapour produced can directly escape so that no significant vapour pressure is capable of building up.
A preferred embodiment of the arrangement in accordance with the invention has been given the further characteristic that the ducts extend between the surfaces lying against one another in the sealing area.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the ducts in a seal of the overlapping type are in the form of depressions provided in the edge area of the laminate.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the depressions comprise grooves, extending transversely in relation to the longitudinal direction of the seal, which are of a length slightly exceeding the width of the seal.
According to the invention a packing laminate web for production of the packing container has been given the characteristics that one edge area of the web is provided with a series of depressions.
Further embodiments of the packing laminate web in accordance with the invention have been given the characteristics which are evident from reading the specification and claims.